The invention relates to catalysts for the process of oxidizing ammonia.
At present, platinum and its alloys with palladium and rhodium are the main industrial catalysts for oxidizing ammonia. Taking the high cost of metals of the platinum group into account, the problem of how to reduce the use and losses of platinoids becomes important. Several compositions of oxide catalysts have been developed in which ferric oxide, chromium oxide, cobalt oxide, or bismuth oxide are the active component.
An oxide catalyst is known which is prepared by mixing aluminum, ferric, calcium and chromium oxides, and subsequently tableting and calcining at 875-900xc2x0 C. (SU N 641985, 1979).
A catalyst for oxidizing ammonia is also known which comprises, % by weight, 90-95 ferric oxide and 5-10 chromium oxide, and is prepared by mixing ferric nitrates and chromium, curing at 315xc2x0 C., cooling, mixing with graphite and subsequently tableting and calcining at 560-650xc2x0 C. (Patent FR No. 2119121, 1972).
An oxide catalyst is known, which is prepared in the form of tablets and consists of ferric oxide and aluminum oxide (SU No. 1220193, 1986). The method for preparing the catalyst consists of mixing ferric oxide and aluminum hydroxide in an acidic medium, with subsequent thermal decomposition of the catalytic mass at 600-700xc2x0 C., grinding, tableting and caking the prepared tablets. Its serviceability in the second step of ammonia oxidation was shown in pilot tests.
Drawbacks of such catalysts are a substantial gas dynamic resistance of the catalyst layer, a change in the phase composition during service, and the presence of ammonia in the gas flow after the catalyst layer.
A catalyst having a perovskite structure for the selective oxidation of ammonia into nitrogen oxide with a yield of at least 90% is selected as the prototype (U.S. Pat. No. 4,812,300, 1989). Powders of perovskites of the series La1xe2x88x92xSrxMeO3, where Me=Co, Mn, and x=0.25-0.75, and LaMeO3, where Me=Co, Mn, Ni, Cr, Fe, were obtained by the method of coprecipitating tetraethyl ammonium from diluted solutions of nitrates taken in appropriate ratios and tested in a reaction of ammonia oxidation (samples were 0.005-0.1 g) at temperatures of from 500 to 1050 K and rates of the gas flow (1.5% by volume of NH3, 5% by volume of O2, balance helium) from 1000 to 10000 h1. However, in order to be used for industrial purposes, the catalyst should be formed into granules. The powders obtained by the coprecipitation method are not only not formed into honeycomb structures, but not into simple granules either.
The object of the present invention is to develop a high-strength, granulated catalyst, including a block catalyst having a honeycomb structure, for oxidizing ammonia, the catalyst having high activity, resistance to thermal shocks, not containing noble metals and making it possible to enhance the safety of the process by reducing the hydraulic resistance of the catalyst layer and stabilizing the gas flow.
The object is attained by using a catalyst in the reaction of oxidizing ammonia, which is a makeup of mixed oxides of the general formula (AxByO3z)k (MemOn)f, wherein: A is a cation of Ca, Sr, Ba, Mg, Be, La or mixtures thereof, B is cations of Mn, Fe, Ni, Co, Cr, Cu, V or mixtures thereof, x=0-2, z=0.8-1.7; MemOn is aluminum oxide and/or oxide of silicon, zirconium, chromium, aluminosilicates, oxides of rare earth elements (REE) or mixtures thereof, m=1-3, n=1-2, k and f are % by weight, with the ratio f/k=0.01-1.
Pure oxides and also an undivided mixture (metal mixture) of oxides of rare earth elements of the following composition, % by weight, may be used as oxides of REE: lanthanum oxidexe2x80x9430-33, ceryl oxidexe2x80x9445-55, praseodymium oxidexe2x80x945-6, neodymium oxidexe2x80x9410-13, samarium oxide mol,xe2x80x941-2, balance lanthanidesxe2x80x94not more than 1. Zirconium oxide additionally comprises an alkaline-earth metal, wherein the ratio of zirconium to the alkaline-earth metal in zirconium oxide is 9:1.
The catalyst may be made up of granules of different configuration, including blocks of honeycomb structure.
The method of preparing the catalyst consists of the following steps:
1) Preparing a powder of AxByO3z oxide.
Oxides of the AxByO3z series are prepared by the method of mechanochemical synthesis (L. Isupova, V. Sadykov, L. Solovyova / Monolith Perovskite Catalysts of Honeycomb Structure for Fuel Combustion // Scientific Basis for the Preparation of Heterogeneous Catalysts. 6th Int. Symp., Louvain-la-Neuve (Belgium), 1994, v. 2, p. 231]. In order to do this, the mixture of initial simple oxides, hydroxides or carbonates, taken in appropriate ratios, is subjected to mechanochemical activation, and then the powder is calcined at 600-800xc2x0 C. for 2-4 hours. The specific surface of the prepared complex oxides is 10-20 m2/g.
2) Mixing and forming.
Powder of the oxide according to 1) is mixed in a powder mixer with compounds providing, during calcining, oxides of Al, Cr, Si, Zr, REE or with aluminosilicates in an acidic medium. In order to enhance the resistance of the catalyst to thermal shocks, reinforcing aluminosilicate fibers may be additionally introduced into the composition of the paste during the mixing step. Used as the Al-comprising adhesive agent are: aluminum oxynitrate or pseudoboehmite; Cr-comprisingxe2x80x94chromic acid; Si-comprisingxe2x80x94kaolin; Zr-comprisingxe2x80x94zirconium oxynitrate; Ln-comprisingxe2x80x94lanthanide nitrates. In order to enhance the rheological characteristics of the paste, surfactantsxe2x80x94ethylene glycol, polyethylene oxide, carboxymethyl cellulose, polyvinyl alcohol, glycerin, etc.xe2x80x94are added to the composition. Granules or blocks of honeycomb structure are formed from the obtained paste.
3) Thermal treatment.
The catalysts are dried at a final temperature of 120xc2x0 C., then calcined in air at 700-1100xc2x0 C. for 2-4 hours.